Electronic systems are typically packaged by mounting individual components such as integrated circuits (ICs) on printed circuit boards. A number of circuit boards are then housed in an enclosure, typically by placing them in slots spaced parallel to one another. Electrical interconnections between the circuit boards are provided by a mother board, sometimes called a backplane, which is disposed perpendicular to corresponding ends of the slots; in certain configurations, integrated circuits may also be mounted on the mother board.
This packaging arrangement is used for computer systems ranging from the smallest personal computer to the largest mainframe. There are several reasons for its popularity, but chief among them seems to be that optional features may be added by simply inserting additional circuit boards into empty mother board slots.
Various schemes are used for cooling a system packaged in this way. Most high performance components cannot cool themselves adequately by connection alone, and thus require an auxiliary cooling apparatus of some sort. The typical cooling apparatus is an air-mover such as a blower or fan positioned to force ambient temperature air across the circuit boards. By orienting the air-mover and circuit boards so that the ambient air is presented to all of the circuit boards in parallel, maximum component reliability is achieved.
A commonly perceived problem with this parallel airflow scheme is that it requires relatively large volumes of air, which, in turn, means that it is fairly noisy. Certain schemes are presently used which reduce the required air volume, such as by reusing the exhaust air. For example, see U.S. Pat. No. 4,837,663 issued to Zushi, et al. in which draft ducts are used to divide and re-mix the air flow between two enclosures, and see U.S. Pat. No. 4,502,100 to Greenspan, et al. in which baffles redirect the air exiting from one side of the circuit boards to their opposite sides.
Unfortunately, by reusing exhaust air which is of a higher temperature than the ambient air, at least some of the components are operated at a higher temperature, and overall system reliability is reduced.
In addition, some have proposed schemes which dynamically control the speed of the air-mover, as in U.S. Pat. No. 4,669,025 to Barker et al. and assigned to Digital Equipment Corporation. The approach shown in that patent adjusts the air flow in response to environmental changes such as ambient temperature, air density, and altitude. It is not directly applicable to minimizing the overall air flow while maximizing component reliability, however.
What is needed is a parallel air flow system wherein the reliability of the system is maximized by presenting ambient temperature air to all circuit boards in parallel, and wherein the required air flow is reduced when the system configuration is reduced, thereby keeping the audible noise to a minimum.